The present invention concerns means for supplying a current. More precisely, it concerns means for supplying a high prevision current, to an external element intended to be connected to such means.
Conventionally, there exist various types of current supply means. Moreover, it will be noted that the present description does not concern what are commonly called current sources.
FIG. 1 shows a circuit including a first example of conventional current supply means 1 intended to be connected, via a connecting line 5 to an element 3 which is external to such circuit. Means 1 are arranged to supply element 3 with a current I1 having a predetermined desired value or nominal value, this value being designated by the reference I1o. For this purpose, means 1 include an operational amplifier A1, and a field effect transistor T1. Means 1 further include integrated resistors such as field effect transistors having an ohmic response, the reference Rint designating the resistor equivalent to the totality of these integrated resistors. Typically, the different components of means 1 are made by a CMOS type manufacturing process widely used in the semi-conductor industry. It goes without saying that these components also include a terminal for connection to a voltage source (not shown) arranged for supplying a supply voltage Vdd to these components.
Transistor T1 made via a process of the aforementioned type, typically includes a drain terminal D, a source terminal S and a gate terminal G. Terminal D of transistor T1 is connected to external element 3 by line 5, and terminal S of transistor T1 is connected to one of the terminals of resistor Rint. Furthermore, operational amplifier A1 typically includes an inverting input terminal, a non-inverting input terminal and an output terminal. The inverting input terminal of operational amplifier A1 is connected to voltage supply means (not shown) arranged for supplying a reference voltage Vref, its non-inverting input terminal is connected to terminal S of transistor T1, and the output terminal of operational amplifier A1 is connected to terminal G of transistor T1.
Essentially, following powering the circuit shown in FIG. 1, the latter becomes stable when the voltage at the non-inverting input terminal of operational amplifier A1 (i.e. the voltage at source terminal S) is substantially equal to that at the inverting input terminal of operational amplifier A1 (i.e. reference voltage Vref). In this case, the output voltage of operational amplifier A1 is substantially constant, so that said voltage provided to terminal G of transistor T1 maintains current I1, which flows through this transistor T1, equal to its nominal value.
The circuit shown in FIG. 1 allows the value of current I1 to be trimmed to its nominal value. The practical realisation of the various components of this circuit inevitably leads to variations in technological parameters, in particular the value of internal resistor Rint which varies by up to +30% with respect to the desired value thereof. Such variations cause current I1 to be provided at a different value from its nominal value.
In order to overcome these ill-timed variations, one then measures the value of current 1 provided to means 1 to which the integrated resistors are connected which are initially short-circuited by connecting lines, as is shown in FIG. 1. Next, certain of these connecting lines are cut by a laser beam, which connects the integrated circuits initially short-circuited by said lines to means 1. This has the effect of increasing the value of resistor Rint connected in series with transistor T1, i.e. of modifying the value of current I1. Such trimming is performed until the value cf current I1 is equal to its nominal value.
One drawback of the current supply means shown in FIG. 1, lies in the fact that it requires the making of a plurality of trimming elements, which is contrary to the usual concerns of the semi-conductor industry as to complexity, space requirement and cost.
Another drawback of the current supply means shown in FIG. 1 lies in the fact that the trimming can be irreversibly performed, so that these means are only suitable for the external element to which means 1 were connected during said trimming.
In order to overcome this drawback, FIG. 2 show a circuit including a second example of conventional current supply means 6. It will be noted that this circuit is similar to that shown in FIG. 1. Thus, the components shown in FIG. 2 and designated by the same references as those shown in FIG. 1, are identical to those shown in FIG. 1.
However, means 6 are connected to a resistor Rext external to said means. Resistor Rext is connected between terminal S of transistor T1 and earth. U.S. Pat. No. 5,291,123 discloses an electric diagram of the same type as that of the circuit described in relation to FIG. 2.
Like resistor Rint described in relation to FIG. 1, resistor Rext shown in FIG. 2 allows current I1 to be trimmed tc its nominal value. For this purpose, the value of current I1 is first determined prior to being provided by means 1 of the circuit shown in FIG. 1. Assuming that voltage Vref is determined as a function of the choice of operation amplifier A1, and that the circuit is stable when the voltage at the non-inverting input terminal of said amplifier (i.e. the voltage equal to the product of the resistance value of resistor Rext by current I1) is equal to the voltage at its inverting input terminal (i.e. voltage Vref), the value of resistor Rext can be determined as follows: ##EQU1##
The value of external resistor Rext intended to be connected to means 6 is thus determined, this connection having to have the effect of trimming the value of current I1 to its nominal value.
One drawback of the current supply means shown in FIG. 2 lies in the fact that it requires the making of a resistor Rext having a low resistance value, in the event that the value of current I1 to be provided must be high. Assuming that supply voltage Vdd is known and constant, the voltage across terminal D of transistor T1 and earth is thus determined and substantially constant. Consequently, a high value of resistor Rext has the effect of reducing the voltage across terminal D of transistor T1 and terminal S thereof, since resistor Rext is connected in series with external element 3 and transistor T1. It is thus necessary to increase the active surface dimensions of transistor T1 so that current I1 which flows through it is equal to said predetermined value.
Those skilled in the art will note that the implementation of a resistor Rext having a low resistance value (typically of the order of several ohms) is costly, in particular in the event that one wishes this resistor to have ar accuracy of the order of +5%.
It will be noted therefore that such a solution does not answer the conventional criteria in the semi-conductor industry as to complexity, space requirement and cost.